Optical fiber transmission systems are being extensively used in the telephone network for long distance and interoffice trunk lines because of their wide bandwidth, small size and insensitivity to electrical interference. Conventional long distance optical transmission systems utilize time division multiplexed digital transmission. The maximum data rate available in commercial lightwave systems was for many years limited to 565 megabits per second and has recently been increased to 1.7 gigabits per second.
Wide transmission bandwidth and high receiver sensitivity can be realized with coherent, or heterodyne, optical communication systems using multiple modulated optical carriers which are closely spaced in frequency. Coherent systems with multiple optical carriers have been disclosed by Shikada in "Multiplex Transmitting Method for Optical Heterodyne/Homodyne Detection Wavelength", Japanese Patent Publication No. 62-43231, 1987. In the Shikada system, one information channel is transmitted on each optical carrier, and N optical carriers can be utilized.
In a typical optical heterodyne communication system, the transmitted optical signal is formed by combining modulated optical carriers originating from plural lasers which emit optical carriers at different optical frequencies. Each laser is directly modulated by an information signal. Alternatively, separate modulators may be used to modulate the respective optical carriers. The modulated carriers are combined in an optical combiner to form an optical signal. The optical signal is then transmitted to a remote receiver over an optical fiber. The received optical signal is heterodyned with a local oscillator lightwave of a different frequency to provide an intermediate frequency signal. The resulting intermediate frequency signal is passed through a filter that selects a desired optical channel from the optical signal, while attenuating the other optical channels. The filtered signal is electronically processed to extract the information signal transmitted on the selected optical channel.
In order to achieve efficient use of the available bandwidth and to avoid interference, or crosstalk, between adjacent channels, optical channels must be carefully assigned. FIG. 1a is an optical frequency spectrum which illustrates a current approach to optical channel spacing in optical heterodyne communication systems. Successive channels i and i+l are separated from each other by a frequency difference .DELTA.f.sub.c. Optical channels i and i+l are shown in FIG. 1a relative to local oscillator frequencies f.sub.LO (i) and f.sub.LO (i+l), which are generated by the local oscillator in the receiver to select optical channels i and i+l, respectively. It is to be understood that the local oscillator generates only one local oscillator frequency at a time.
When optical channel i is the selected channel, the optical signal in channel i centered on frequency f.sub.s is heterodyned to an intermediate frequency f.sub.IF which is equal to f.sub.s -f.sub.LO. The optical channel spacing must be selected so that the level of crosstalk between the selected channel and adjacent channels is acceptable for a given application.
FIG. 1b depicts the intermediate frequency spectrum resulting from heterodyning. The heterodyning of the local oscillator lightwave with the received optical signal results in positive and negative frequency images of each of the optical channels. The positive image of the optical channel i is centered at the intermediate frequency f.sub.IF. The corresponding negative image is centered at frequency -f.sub.IF. Likewise, the positive and negative images of channel i-l result from heterodyning as shown in FIG. 1b. In particular, the positive image of channel i-l is positioned in a frequency range adjacent to the positive image of channel i. To prevent an unacceptable level of interference between adjacent channels, the channel spacing is selected such that adjacent channels do not overlap after heterodyning. Typically, the channel spacing .DELTA.f.sub.c is selected so that EQU .DELTA.f.sub.c .gtoreq.2f.sub.IF +X,
where X is the bandwidth of the optical channels. In order to prevent channel overlap after heterodyning, the channel spacing .DELTA.f.sub.c must satisfy the above inequality.
The above approach is inefficient in its use of the available spectrum. In particular, there are unused portions of the spectrum in such systems. This inefficiency limits the number of optical channels that may be assigned to a given frequency range. The maximum number of channels that can be heterodyned is limited by the continuous tuning range of the local oscillator laser in the receiver. The tuning range of different laser types varies greatly and is small for some commercially available laser types. It is therefore desirable to provide techniques for reducing channel spacing in optical heterodyne communication systems, thereby facilitating use of less sophisticated lasers as local oscillators and increasing the information-carrying capability of such systems.
Image rejection receivers, wherein one of the signal sidebands is suppressed, permit closer optical channel spacing. However, image rejection receivers are relatively complex and expensive.
It is a general object of the present invention to provide improved optical communication systems.
It is another object of the present invention to provide optical heterodyne communication systems wherein the average channel spacing is reduced in comparison with prior art systems.
It is another object of the present invention to provide compressed optical channel spacing in an optical heterodyne communication system.
It is a further object of the present invention to provide an optical heterodyne system with compressed channel spacing using conventional optical communication equipment.
It is another object of the present invention to provide compressed optical channel spacing without increasing interference from adjacent channels.